This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge ignited by a high voltage pulse. The invention further relates to a method of making photoflash units using such lamps.
High voltage flashlamps may be divided historically into three catagories: (1) those having a spark gap within the lamp such that electrical breakdown of a gaseous dielectric (e.g., the combustion-supporting oxygen atmosphere) is an integral part of the lamp ignition mechanism; (2) those having a conductive primer bridge that electrically completes the circuit between the lead-in wires; such primers are rendered conductive by additives such as acetylene black, lead dioxide, or other electrical conduction-promoting agents; and (3) lamps having an essentially nonconducting primer bridge that connects the inner ends of the lead-in wires and which becomes conductive, upon application of a high voltage pulse, by means of breakdown of the dielectric binder separating conductive particles therein.
The earliest high voltage flashlamps were of the spark gap type construction wherein an electrical spark would pass through the gaseous atmosphere within the lamp. The spark would jump between two electrodes, at least one of which was coated with a primer composition. Such lamps tend to exhibit poor sensitivity and reliability when flashed from low power sources such as the miniaturized piezoelectric devices that are suited for incorporating into pocket-sized cameras. Most of the electrical input energy in such lamps is lost to the gas atmosphere by the spark. Also, the electrical characteristics vary considerably from one lamp to another because of shreds of metallic combustible in the spark gap and consequent variations in effective gap length.
The use of spaced lead-in wires interconnected by a quantity of electrically conductive primer gives rise to highly predictable behavior and a well-defined electrical path through the lamp. Here again, however, relatively high-powered flash sources must be used in order to attain reliable lamp flashing.
Present state of the art flashlamps of the high voltage type make use of a bridge of initially nonconducting primer to interconnect the inner ends of the lead-in wires. Considerably higher sensitivity is attainable by this method, apparently because the breakdown and discharge follow a discrete path through the primer composition and thereby promote greater localized heating. With respect to specific construction, such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more sleeves or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as zirconium or hafnium, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres. The outer surface of the lamp envelope is generally covered with a protective reinforcing coating such as cellulose acetate.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, as, for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
The primers used in such flashlamp are designed to be highly sensitive toward high-voltage breakdown. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamp. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern, miniature cameras. The mechanical energy delivered to the piezoelectric crystal and thereby the electrical output energy therefrom is limited both by the size of the device and by the necessity to minimize camera vibration and motion during use.
The high degree of electrical sensitivity needed in high-voltage flashlamps gives rise to distinct problems of inadvertent flashing during their manufacture, lacquer coating, and subsequent handling. Any static charges on equipment or personnel can cause these lamps to flash. Some such lamp flashes even occur when the lamps are lying stationary in an isolated spot. Apparently, even air movements can generate sufficient electrostatic energy to promote flashing of those lamps that are by nature the most sensitive and susceptible. This problem is greatly compounded by the fact that flashlamps flash sympathetically, i.e., the radiant energy from one lamp that flashes is sufficiently intense to ignite the shredded combustible in adjacent lamps. During lamp manufacture on modern high-speed equipment, it is necessary, or at least high expedient, at certain stages of processing to accumulate the lamps in containers, having from about 30 to more than 2,000 lamps in a container. The problem that is encountered is that should one lamp be inadvertently ignited, all lamps in that container will sympathetically flash and be lost.
It is common practice in photoflash lamp manufacturing to dip the lacquered lamps into a bath which leaves a film of antistatic agent on their surfaces. This does much to prevent buildup of an electrostatic charge on a lamp itself by rubbing or handling. It does not, however, give a significant protection for the lamp against contact with external charges.